In accordance with well known physics, the Doppler effect is a frequency shift that occurs to a signal as it propagates from a source to a receiver, where the source and receiver are in motion with respect to each other. It is a well known effect in both acoustics and in the electromagnetic frequency spectrum. The Doppler effect is exploited by systems such as Doppler Radar, which use the phenomena to measure the velocity of a target.
In electromagnetic environment simulation systems, the radio frequency (RF) emissions of a vast number of RF emitters are simulated to replicate the electromagnetic environment of a modern battlefield. The RF emitters may simulate fixed or moving (airborne) radar installations, as viewed from an airborne surveillance system (Own Ship). Systems of this type include the AAI Advanced Standard Threat Generator (ASTG), the Northrop Grumman Combat Electromagnetic Environment Simulator (CEESIM), and the AAI Advanced Architecture Phase Amplitude Time Simulator (A2PATS).
Doppler effects are computed from the radial velocity between the emitter and the receiver:Doppler_Freq(one-way)=−Rad_Vel/λ
Where: Doppler_Freq (one way) denotes the frequency shift, in units of Hertz, that is caused by one-way propagation from an emitter to a receiver, due to relative motion between them. Radar problems involve 2-way propagation, so the radar equation for Doppler has an additional factor of 2.
Rad_Vel denotes the radial velocity between the emitter and receiver, in units of distance per unit time (e.g., ft/sec). Radial velocity is defined as the time rate of change of the slant range from the receiver to the emitter. Positive radial velocity indicates increasing, or opening, range. Positive Doppler is induced by closing (negative) radial velocity.
λ denotes the emitter carrier wavelength, in units of length (e.g., ft) similar to the units for radial velocity.
Multiple emitters, as viewed by a common receiver, typically each have their own Doppler shift, because the emitters typically have different wavelengths and different radial velocity.
Doppler frequency shift is typically applied as a quasistatic frequency offset to the emitter carrier frequency. In addition, since there is a relatively large amount of 3D vector mathematics involved in computing the slant range and radial velocity (hence, the Doppler), an update rate is limited by the amount of computing power required to perform each update. Doppler frequency update rates are usually less than 100 Hz (100 times a second), and even the highest fidelity implementations are less than 1 KHz.
One of the problems with prior implementations is their fundamental inability to deal with new requirements for high fidelity Doppler simulation. New digital receivers have advanced geo-Location abilities. In order to test these next-generation systems, electromagnetic environment simulators are required to simulate Doppler effects at a much higher level of fidelity than ever before. In particular, there is a requirement to accurately simulate differential effects as the aircraft maneuvers and accelerates.
The implications of producing a significantly higher fidelity Doppler simulation are twofold: First, Doppler must be treated not as an offset frequency, but rather as a continual accumulation of differential phase. The rate of phase accumulation (frequency) must slew in accordance with radial acceleration, thereby exactly mimicking the real-world Doppler phenomena. Second, the Doppler update rate must be much, much faster than legacy implementations (i.e., tens of KHz), in order to faithfully replicate the phenomenology. This leads to a huge issue of how to compute the 3D vector mathematics at the required rate.